Methods for cleaning microelectronic substrates using ultradilute cleaning liquids

ABSTRACT

A method of cleaning a surface of an article using cleaning liquids in combination with acoustic energy. Preferably, an ultradilute concentration of a cleaning enhancement substance, such as ammonia gas, is dissolved in a liquid solvent, such as filtered deionized water, to form a cleaning liquid. The cleaning liquid is caused to contact the surface to be cleaned. Acoustic energy is applied to the liquid during such contact. Optionally, the surface to be cleaned can be oxidized, e.g., by ozonated water, prior to cleaning.

This application is a continuation-in-part application of U.S. patentapplication Ser. No. 09/311,800 filed May 13, 1999 in the names of Puriet al. , which application is incorporated herein by reference in itsentirety.

FIELD OF THE INVENTION

The present invention relates to methods of cleaning the surfaces ofarticles, particularly microelectronic devices at one or more pointsduring the manufacture of such devices. More particularly, the presentinvention relates to methods of cleaning surfaces of microelectronicdevices using wet processing techniques in conjunction with theapplication of acoustic energy and/or in conjunction with an oxidizingpre-treatment. In preferred embodiments, cleaning liquids of the presentinvention are ultradilute solutions formed by dissolving a gas solute ina suitable solvent.

BACKGROUND OF THE INVENTION

Since the early days of the microelectronic industry, the importance ofminimizing contamination on microelectronic devices throughout themanufacturing process has been recognized. Contaminants includeparticles, photoresist residue, and/or the like whose presence canadversely impact the performance and function of microelectronic devicesif not adequately removed. Accordingly, various cleaning treatments havebeen devised.

However, as the end product devices have become more and moreminiaturized, a contaminant occupies an increased percentage of theavailable space for circuitry and other device elements. Hence,cleanliness of the materials has become far more critical, andcleanliness specifications have become increasingly more stringent.Unfortunately, these trends make cleaning much more challenging.

One traditional cleaning approach has involved the use of an aqueoussolution of H₂O₂ and NH₄OH to carry out cleaning in which the volumeratio of the peroxide to aqueous ammonia to water typically is 1:1:5.These relatively concentrated mixtures are commonly referred to as theStandard Cleaning Solution 1 (“SC-1”). The SC-1 process is believed todetach particles through surface etching. The use of megasonic energy incombination with the SC-1 chemistry is reported to improve particledetachment.

In basic solution, many common particle types will bear the same kind ofsurface charge as do silicon and silicon dioxide surfaces being cleaned,theoretically causing the particles and the surfaces to repel each othervia electrostatic repulsion. However, SC-1 cleaning solutions tend tohave very high ionic strength, causing the electrostatic repulsionforces to be negligible as a practical matter. Electrostatic repulsionforces therefore play only a minor role, if any, in connection with theSC-1 chemistry.

Although widely used, the SC-1 approach has drawbacks. These relativelyconcentrated solutions might clean effectively, but unfortunately theycan also deposit metal contaminants onto the devices being cleaned. Ofcourse, a cleaning method that deposits contaminants iscounterproductive. These concentrated solutions can also unduly etch anddamage the surfaces of the devices being cleaned. Device damage is alsoa result that is desirably avoided by a cleaning method. As stillanother drawback, the peroxide typically must incorporate stabilizers,and these can contaminate the surface being cleaned, which also iscounterproductive especially when relatively high concentrations ofperoxide are used.

Another cleaning approach is described in U.S. Pat. No. 5,656,097. Thisapproach involves cleaning devices with aqueous solutions of ammonia andhydrogen peroxide in combination with the application of megasonicenergy. In this approach, the dilute solutions are prepared by dilutingmore concentrated solutions of aqueous ammonia with water. The approachhas drawbacks. Although dilute, these solutions can still unduly etchthe devices being cleaned, metal contaminants can still be deposited,and the stabilizer for the peroxide is still a contaminant. Further, itis very difficult to prepare dilute solutions with good accuracy bydiluting relatively small volumes of concentrated solutions withrelatively large volumes of solvent. The inaccuracy can lead todifferences in cleaning performance from device to device.

It can be seen, therefore, that improved cleaning methods that cansatisfy these more stringent demands imposed by miniaturization arestill needed.

SUMMARY OF THE INVENTION

The present invention provides an approach for cleaning articles, suchas microelectronic devices at various stages of manufacture, that isextremely effective at removing particle contaminants and/or organicdebris, e.g., photoresist remnants, from the device surfaces. Inpreferred embodiments, the approach accomplishes cleaning withoutdepositing metal contaminants onto the surface of the devices andwithout undue etching or other damage of the surfaces. As used herein,the term “microelectronic device” includes but is not limited tosemiconductor wafers, integrated circuits, thin film heads, flat paneldisplays, microelectronic masks, and the like. The term shall also referto partially completed devices as they are being manufactured.

The approach of the present invention is significant in at least tworespects. First, preferred embodiments of the present invention achievecleaning efficiencies of better than 99.9% with respect to particleshaving a size greater than about 0.16 microns. Second, in addition tohigh particle removal efficiency, it is also important to carry outcleaning operations without adversely affecting surface smoothness andwithout depositing additional contaminants, e.g., metal contaminants,onto the surface being cleaned. Preferred embodiments of the presentinvention do not adversely affect surface roughness in any significantway. In fact, preferred cleaning embodiments of the present inventionhave actually provided post-clean surfaces that are smoother than thesame pre-cleaned surfaces Metal contamination of preferred embodimentsis neutral, meaning that cleaning operations deposit substantially no,if any, metal contaminants onto the surfaces being treated.

Preferred embodiments of the invention carry out cleaning operationsusing the combination of acoustic energy and a cleaning liquidcomprising an ultradilute concentration of a cleaning enhancement agent.Amazingly, the combined use of acoustic energy, particularly megasonicenergy, and ultradilute cleaning reagents provides exceptional cleaningperformance even though the amount of cleaning enhancement agent in thereagent is almost negligible as a practical matter. For example,reagents containing approximately 100 ppm gaseous anhydrous ammoniadissolved in filtered deionized water remove particles from substratessuch as semiconductor wafers with very high efficiency.

In one aspect, the present invention provides a method of cleaning asurface of an article using ultradilute cleaning liquids in combinationwith acoustic energy. An ultradilute concentration of a cleaningenhancement substance, such as ammonia gas, is dissolved in a liquidsolvent, such as filtered deionized water, to form a cleaning liquid.The cleaning liquid optionally may also include other ingredients, suchas hydrogen peroxide, if desired, but such additives are not needed andmay not even be desired to achieve excellent cleaning performance. Thecleaning liquid is caused to contact the surface to be cleaned. Contactcan occur by causing the liquid to flow past the surface, by sprayingthe liquid onto the surface, by submerging the surface in a body of theliquid, and/or the like. Preferably, acoustic energy is applied to theliquid during such contact.

In another aspect, the present invention provides a cleaning method inwhich a surface of an item to be cleaned is first contacted with aprocessing liquid comprising an oxidizing agent. A preferred processingliquid for this purpose is ozonated water, but solvents such as watercontaining other oxidants such as hydrogen peroxide could also be usedif desired. Next, the surface is contacted with a cleaning liquid,preferably ultradilute aqueous ammonia. Acoustic energy is directed intothe cleaning liquid during at least a portion of the time that contactwith the cleaning liquid is occurring.

In another aspect, the present invention provides a cleaning method inwhich a substrate is positioned in a cleaning vessel with the surface tobe cleaned being substantially vertical. A cleaning liquid comprisingand ultradilute concentration of a cleaning enhancement substance,preferably ammonia, is then introduced into the vessel. As the vesselfills, the rising top surface of the cleaning liquid traverses thesubstrate surface. Acoustic energy is applied to the rising cleaningliquid.

In still another aspect, the present invention involves a method ofcleaning a surface of an article in which an ultradilute concentrationof a gaseous cleaning enhancement substance is dissolved in a liquidsolvent to form a cleaning liquid. The cleaning liquid is caused tocontact the substrate surface. While causing the cleaning liquid tocontact the substrate surface, acoustic energy is applied to thecleaning liquid. After causing the cleaning liquid to contact thesubstrate surface, the substrate surface is rinsed and then dried.Preferably, drying occurs by contacting the substrate surface with afirst process reagent comprising a carrier gas, preferably nitrogen, anda cleaning enhancement substance, preferably an ultradiluteconcentration of isopropyl alcohol. The substrate surface is alsocontacted with a drying reagent comprising a heated gas, preferablyheated nitrogen.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic flow chart of a preferred system for practicingthe principles of the present invention.

FIG. 2 is a schematic diagram of a single tank system of the presentinvention suitable for pretreating devices with ozonated water and thenfollowing up by cleaning the devices with ultradilute, aqueous ammonia.

FIG. 3 is a schematic diagram of a twin-tank system of the presentinvention in which the system includes cleaning capabilities inaccordance with the present invention as well as rinse/dry capabilities.

DETAILED DESCRIPTION

For purposes of illustration, the principles and methods of the presentinvention will now be described in connection with system 10 shown inFIG. 1. One or more substrates, such as semiconductor wafer 12, arepositioned inside cleaning vessel 14. In a preferred orientation whenprocessing liquids are introduced into cleaning vessel 14 from thebottom (as is shown), wafer 12 is positioned so that the surface 16 ofwafer 12 to be cleaned is oriented substantially vertically duringcleaning operations. Substantially vertically means that the surface istilted at an angle in the range from 0 degrees to about 10 degrees. Forplanar substrates such as semiconductor wafer 12, a slight tilt awayfrom vertical is desired in order to help prevent adjacent substratesfrom being jostled against each other, or against any carrier (notshown) in which the wafers are held, as processing fluid flows past thesubstrates. Such tilting is optional and may not be desirable in someprocesses. For example, 200 mm wafers need not be tilted at an angle dueto their mass, but 150 mm wafers can advantageously be tilted at anangle of 7 degrees to 8 degrees. This principle of supporting planarsubstrates at a slight tilt away from vertical is described further inU.S. Pat. No. 5,571,337.

A flow of cleaning liquid from cleaning liquid source 18 is introducedinto cleaning vessel 14 through inlet end 20 and is discharged throughoutlet end 21. Cleaning vessel 14 preferably is made of material that isas transmissive as practically possible to acoustic energy so thatacoustic energy from acoustic energy source 22 (described further below)is applied to the contents of cleaning vessel 14 with minimal energyloss due to reflection or absorption of the energy by the walls ofcleaning vessel 14. The material used to form cleaning vessel 14 shouldalso shatter resistant and strong enough so that the material resistscracking, chipping, or shattering during use. Examples of such materialsinclude PTFE available under the trade designations Teflon or Hylar),quartz, combinations of these, and the like. Of these, quartz ispreferred. When processing liquids are introduced into cleaning vessel14 through inlet structure (not shown in FIG. 1) such as a sparger,manifold, other flow distribution device, or the like, such inletstructure, to the extent that it is in the path of acoustic energydirected into cleaning vessel 14 also is made of an acoustic energycompatible material such as quartz.

Cleaning vessel 14 can be any kind of cleaning vessel that allows a flowof cleaning fluid to be maintained past wafer 12. For example, cleaningvessel 14 can be the cascade, overflow vessel incorporated into theSeries 6000 equipment commercially available from Yield UpInternational, Inc., Mountain View, Calif.

As the cleaning liquid flows through cleaning vessel 14, the liquidcontacts and helps to clean, e.g., remove particles and/or otherimpurities, from surface 16 of wafer 12. The cleaning liquids of thepresent invention generally include a solvent and cleaningly effectiveamounts of one or more cleaning enhancement substances. Examples ofsolvents that can be used in the practice of the present inventioninclude deionized water as well as isopropyl alcohol, ethanol, methanol,other polar organic solvents. Of these, deionized water is mostpreferred. Examples of cleaning enhancement substances include NH₃,NH₄OH, HCl, HF, ozone, and the like. Generally, acid cleaningenhancement substances are suitable for cleaning metal surfaces, whereasbasic cleaning enhancement substances such as ammonia are suitable forcleaning nonmetallic surfaces such as silicon and silicon oxidesurfaces. For cleaning liquids comprising ultradilute concentrations ofa cleaning enhancement substance in a solvent (described further below),it is preferred that the cleaning enhancement substance be a gaseoussolute dissolved in the solvent. When the solute is supplied as gas tobe dissolved into the solvent, the cleaning liquid can be formed bymixing appropriate flows of the solvent and the gas. This allows theultradilute concentration of the solute in the solvent to be controlledwith great accuracy. Gases can also be filtered to a degree not possiblewith liquids, meaning that the gas solute can be provided with a highlevel of purity.

In the practice of the present invention, any deionized waterincorporated into any processing liquid that is caused to contact wafer12 preferably is electrostatically filtered using the Clean Point®filtration unit commercially available from Yield Up International,Mountain View, Calif. This particular filtration unit uses a series ofpositive and negatively charged filters and is extremely effective atremoving extremely particles from process water with only a minimalpressure drop through the filtration unit. The features and operation ofsuch filters are further described in U.S. Pat. No. 5,571,337.

The concentration of the cleaning enhancement substance in the cleaningliquid will depend upon how the principles of the present invention arebeing practiced. For instance, in embodiments of the present inventionin which cleaning operations comprise an oxidizing pretreatment(described below), the concentration of the cleaning enhancementsubstance in the cleaning liquid can vary within a wide effective range.Preferably such concentration is ultradilute. The term “ultradilute”preferably means that the volume ratio of the solvent to the cleaningenhancement agent is in the range from 500:1 to 500,000:1, preferably1000:1 to 300,000:1, more preferably 100,000:1 to 200,000:1. In someinstances in which it may not be practical to determine a volume ratio,then these ratios may be determined on a molar basis.

Surprisingly, it has now been discovered that ultradilute solutions ofcleaning enhancement substances are extremely effective and beneficialcleaning liquids for use in the manufacture of microelectronic devices.The fact that ultradilute solutions have any cleaning effect at all isunexpected, yet such cleaning compositions offer numerous advantages.First, ultradilute solutions cause insignificant, if any, etching of thesurface being cleaned. For example, studies have shown that only a fewangstroms, if even that much, of the native oxide surface is etched whencleaning occurs with an ultradilute cleaning liquid. Thus, a surfacecleaned with an ultradilute cleaning liquid is less affected than if itwere to be cleaned with more concentrated solutions that can etch tens,if not hundreds, of angstroms of the surface during a cleaningoperation.

Second, ultradilute solutions effectively clean an extremely highpercentage of particles from surfaces of microelectronic devices, whichis a result that is counter to conventional wisdom in the manufacture ofmicroelectronic devices. Conventional wisdom has suggested that theactive cleaning agent in a cleaning liquid must be present at highenough concentrations in order to undergo primarily a bulk reaction,e.g., etching, with the substrate surface. The belief has been that asurface must be measurably etched in order to remove particles from thesurface. However, bulk reactions are not always desirable since suchreactions can damage, e.g., unduly roughen, the substrate in ways thatare difficult to control. Surprisingly, ultradilute cleaning liquidsaccomplish removal of particles from substrate surfaces withsubstantially no etching of the substrate surface. As used herein,substantially no etching means that 10 angstroms or less, morepreferably 5 angstroms or less, of the surface is etched by the cleaningliquid. In some instances, we have even observed no measurable etchingof the substrate surface after cleaning.

While not wishing to be bound by theory, a possible rationale to explainthe unexpected cleaning capabilities offered by ultradilute cleaningliquids can be suggested. It is believed that the ultradiluteconcentration of a cleaning enhancement substance alters the pH of thecleaning liquid enough so that the zeta potential characteristics ofparticle contaminants on the substrate surface and/or of the substratesurface itself are altered when the surface is being cleaned with a flowof these solutions. Specifically, it is believed that the zeta potentialalteration causes the substrate surface and the particles to becharacterized by opposite surface charges so that electrostaticrepulsion between the particle contaminates and the surface helps toeject the particles away from the surface where the particles are moreeasily caught up and carried away by the flowing cleaning liquid. Beingultradilute, the solutions have relatively low ionic strength, and theelectrostatic repulsion forces can play a key role in particle removal.Thus, ultradilute cleaning liquids are believed to benefit frominterfacial reactions occurring at the liquid-solid interface betweenthe cleaning liquid, on the one hand, and the substrate surface andparticles, on the other hand. This is a completely different cleaningmechanism than the bulk reactions more characteristic of moreconcentrated cleaning liquids.

Still yet another advantage of using ultradilute solutions is thatcleaning occurs rapidly. For example, five minutes, more preferablythree minutes, is adequate for cleaning the substrate surface 16. Thisshort cleaning duration means fast cycle times. Yet, on the other hand,if it were to be desired for some reason to carry out cleaning for alonger duration, that could be done without damaging substrate surface16 since the cleaning liquid has very little effect upon the substratesurface due to the ultradilute concentration of the cleaning enhancementsubstance.

As another advantage, it is believed that ultradilute solutions depositinsignificant amounts, if any, of metal contaminants onto the surfacesbeing cleaned.

Cleaning liquids may, if desired, be prepared by diluting a concentratedliquid solution of the cleaning enhancement substance with the desiredsolvent. However, when the cleaning liquid contains only an ultradiluteconcentration of the cleaning enhancement substance, it can be verydifficult to accurately control such dilution when the dilution occurscontinuously over a period of time (as opposed to batchwise dilution) tosupport a steady state cleaning operation. As a consequence,unfortunately, the concentration of the cleaning enhancement substancein the diluted cleaning liquid can vary quite a bit. This variation inconcentration may adversely affect the performance of the cleaningoperations. In contrast to the dilution approach, it is substantiallyeasier to establish and maintain a uniform ultradilute concentration ofthe cleaning enhancement substance in the solvent when the cleaningenhancement substance is supplied as a gas. Appropriate flow rates ofthe solvent and the gaseous cleaning enhancement substance are easilyestablished and mixed together to form a cleaning liquid with thedesired ultradilute concentration. A number of gas/liquid mixing devicesare known for accomplishing this. A preferred gas/liquid mixer iscommercially available under the trade designation from Legacy Systems,Inc.

In addition to cleaning liquid source 18, one or more other sources ofprocessing fluids, such as a gas, liquid, slurry, and/or the like, mayoptionally also be fluidly coupled to cleaning vessel 14 so that suchother processing fluids may also be used to treat surface 16 of wafer 12before, during, or after cleaning operations. For purposes ofillustration, FIG. 1 shows system 10 as including two additional sources24 and 26 of processing fluids.

In preferred embodiments, optional fluid source 24 is a source of arinsing fluid that is present so that surface 16 optionally can berinsed before and/or after being treated with the cleaning liquid and/orany other processing liquid. A preferred rinsing fluid is deionizedwater that has been electrostatically filtered using the Clean Point Rfiltration unit.

Optional fluid source 26 is preferably an oxidizing processing liquidcomprising a suitable solvent and an amount of an oxidizing agenteffective to oxidize the surface of the wafer 12 without undulyaffecting its physical (e.g., surface smoothness characteristics) andfunctional characteristics (e.g., electronic characteristics ofoperational structures formed on or in the wafer 12, if any). Such anoxidizing fluid may be used to oxidize surface 16 of wafer 12 beforetreatment of surface 16 with the cleaning liquid from cleaning liquidsource 18. Advantageously, such an oxidizing pre-treatment enhances theeffectiveness of the cleaning operation for at least three reasons.First, the oxidizing pre-treatment and subsequent treatment with thecleaning liquid work together to more effectively clean substratesurface 16, because the oxidizing pre-treatment it oxidizes organicmaterials on the surface, such as photoresist remnants, which may thenbe easier to remove. The oxidizing treatment thus boosts cleaningperformance because it extends the cleaning effect to a class ofmaterials, organic materials, that otherwise might not be effectivelyremoved by the subsequent treatment with the cleaning liquid. Oxidizingtreatment is thus desirable when surface 16 is known to have organiccontaminants such as photoresist remnants.

Second, the oxidizing treatment also forms an oxide barrier layer on thesurface of substrate 12 that protects the substrate from being damagedby the cleaning liquid when such protection may be desired. Such damageis more of a concern when the cleaning liquid might be an etchant of thesubstrate surface being cleaned. For example, silicon can react withaqueous ammonia solutions. In contrast, silicon oxide tends to be lessreactive with such aqueous ammonia solutions. Accordingly, when thecleaning liquid is aqueous ammonia and the substrate surface 16 includessilicon, it can be desirable to oxidize the silicon surface of thesubstrate to form a protective oxide barrier over the silicon beforecontacting the substrate surface 16 with the aqueous ammonia.

Third, the resultant oxide surface tends to enhance the cleaning effectwhen the surface comprises silicon being oxidized to silicon oxide. Inparticular, it has been observed that cleaning a silicon surface withultradilute aqueous ammonia is more effective in conjunction with anoxidized surface. Although the reason for this is not known, it isbelieved that the electrostatic repulsion forces between substratesurface 16 and particle contaminants are stronger when surface 16 isoxidized.

Preferred oxidizing compositions are aqueous solutions containingdeionized water as a solvent and an oxidizing agent selected from ozone,hydrogen peroxide, nitric acid, combinations of these, and the like.Preferably, the oxidizing composition is ozonated water containing anultradilute concentration of ozone. More preferably, the oxidizing agentis ozonated water containing 5 ppm to 100 ppm, preferably 10 ppm to 50ppm, most preferably about 17 ppm ozone. Use of ultradilute ozonatedwater is advantageous because such compositions cause the formation of athin, protective oxide layer without otherwise affecting the wafer 12 inany significant way. Ozonated water also is very easy to purify. Incontrast, oxidizing agents such as hydrogen peroxide are much moredifficult to purify and also contain stabilizers which are themselvescontaminants. Oxidants such as nitric acid are less desirable than ozonein that aqueous nitric acid is also a strong etchant and therefore mayalso tend to etch the substrate too much.

System 10 also includes source 22 of acoustic energy operationallycoupled to cleaning vessel 14 so that acoustic energy can be directedinto cleaning vessel 14. Examples of acoustic energy include sonic,supersonic, ultrasonic, and megasonic energy. The use of megasonicenergy is preferred in that megasonic energy is most effective atremoving smaller particles from substrate surface 16. Equipment forgenerating acoustic energy is commercially available from a number ofvendors. Particularly preferred equipment for applying megasonic energyto cleaning vessel 14 includes a generator model no. 68101 and a series98S or series 7857S transducer, or the like, commercially available fromKaijo Corp. Advantageously, use of this particular equipment allowsacoustic source 22 and cleaning vessel 14 to be made of easily separablestructures so that either unit can be removed for service, replacement,or repair as needed. This significantly lowers the cost of maintainingsystem 10 in that one of the cleaning vessel and acoustic source 22 canbe serviced without touching the other. In contrast, many previouslyknown designs integrate the acoustic source and cleaning vessel so thatindependent service, replacement, and repair are not practical. Such aninterdependent design is more expensive to maintain since replacement ofone part would necessitate replacement of both parts.

In one particularly preferred embodiment of the present invention, thecleaning liquid is most preferably an ultradilute, aqueous ammoniasolution formed by dissolving anhydrous ammonia gas in filtered,deionized water. It has been found that this cleaning liquid providesextremely effective cleaning performance, particularly when the surfaceto be cleaned comprises silicon oxide. In another particularly preferredembodiment of the present invention suitable for cleaning siliconsurfaces, the surfaces are first oxidizing by treatment with ozonatedwater and then treated with a cleaning liquid in the form of anultradilute, aqueous ammonia solution formed by dissolving anhydrousammonia gas in filtered, deionized water. It has been found that thiscombination provides an extremely high level of cleaning performance,particularly when the substrate surface to be cleaned is hydrophobicbefore treatment and/or includes organic contaminants.

In a preferred mode of operation, an optional flow of rinsing liquidfrom source 24 is established through cleaning vessel 14. Substrates 12may be positioned in cleaning vessel either before or after this flow isestablished. The substrates in the case of wafers, masks, disks, flatpanels, liquid crystal displays, thin film heads, photomasks, lenses,and the like, can be face to face, back to back, face to back, or backto face. Face to face and back to back is a preferred orientation. Thisflow may occur at room temperature for 2 to 5 minutes using flow ratesappropriate to the type of equipment being used in accordance withinstructions provided by the vendor. For example, in a cascade rinsevessel of the type incorporated into the equipment sold by Yield UpInternational, Inc., the flow rate in this and all other processingsteps of the preferred mode of operation may be in the range from 0.1 to50, preferably 0.5 to 10, more preferably about 5 gallons per minute.

Optionally, a flow of oxidizing liquid from source 26 may replace theflow of rinsing liquid through cleaning vessel 14 in order to oxidizesubstrate surface 16. This flow may be established with or withoutdumping the rinse liquid first. Preferably, however, the flow ofoxidizing liquid is established without dumping the rinse liquid whenthe oxidizing liquid flow is established. The length of time andtemperature for carrying out this treatment will depend upon factorsincluding the composition of the oxidizing liquid, the nature ofsubstrate surface 16, and the like. For ozonated water containing about5 to 100, preferably 10 to 60, more preferably about 17 ppm ozone,treating wafer 12 for 2 to 5 minutes at room temperature would besuitable. These conditions allow an oxide layer to form on substratesurface having a thickness in the range of 8 angstroms to 11 angstroms.Shorter processing times may be used, but the formation of oxide may beincomplete. Longer times may offer no additional benefit, thusincreasing cycle time and expense without good reason.

Next, wafer 12 is again optionally rinsed with the rinsing liquid,preferably filtered deionized water, in order to wash away the oxidizingliquid. This rinsing step may occur under the same range of processingconditions that are suitable for the first rinsing step noted above,except that the rinsing liquid preferably is heated for this rinse. Ifheated, the temperature of the rinsing liquid preferably may be anytemperature in the range from about ambient temperature to 85° C. Theozonated water of the previous step may be optionally dumped prior tointroducing the rinsing liquid.

Next, a flow of the cleaning liquid through cleaning vessel 14 isestablished. This can be done with or without dumping the rinsing liquidfirst. However, in embodiments in which the cleaning liquid isultradilute with respect to the cleaning enhancement substance, it ispreferred to dump the rinsing liquid first before establishing acascading flow of the cleaning liquid. Dumping may take more time, butcleaning performance is dramatically better. While the reason for thisis not known, a possible rationale can be suggested. When the rinseliquid is dumped, cleaning vessel 14 is empty, at least in the sensethat no part of wafer 12 is submerged in processing liquid when cleaningliquid is introduced into cleaning vessel 14. Accordingly, in acascading rinse cleaning vessel, the top surface of cleaning liquidrises and traverses across surface 16 as cleaning vessel 14 fills. It isbelieved that the zeta potential effects are quite strong at the movinginterface between the liquid surface and the substrate surface 16,facilitating particle removal.

Treatment with the cleaning liquid may be carried out at any convenienttemperature within a wide range. For example, the cleaning liquid can bechilled to any temperature below ambient, but above which the cleaningliquid freezes, at ambient temperature, or heated to a temperature aboveambient but below which the cleaning liquid would boil. For example, foraqueous cleaning liquids comprising ultradilute concentrations ofammonia (i.e., ultradilute, aqueous ammonium hydroxide), the cleaningliquid preferably can be supplied at any temperature or temperatures inthe range from 0° C. to 98° C., preferably 20° C. to about 85° C. Thecleaning liquid and cleaning vessel can be pressurized if it is desiredto expand the temperature range within which the cleaning liquid couldbe supplied in the liquid phase.

Excellent cleaning results are obtained by heating the cleaning liquidto elevated temperatures in the range from about 60° C. to about 85° C.,which are typical temperatures used in connection with the SC-1chemistry. However, unlike SC-1 chemistry, excellent cleaningperformance can be achieved with ultradilute cleaning liquids of thepresent invention at unconventionally low cleaning temperatures. Forexample, the performance of SC-1 chemistry generally is poor at lowtemperatures below about 60° C., particularly below about 30° C. becausethe etching rate of the cleaning liquid slows down too much at suchlower temperatures. In contrast, the cleaning performance of ultradilutecleaning liquids of the present invention is maintained, and is evenimproved in key aspects, as the temperature of the liquid is reduced.Accordingly, preferred embodiments of the invention involve supplyingthe cleaning liquid at a temperature below about 30° C., preferably fromabout 0° C. to about 25° C.

During the cleaning step, acoustic energy, preferably megasonic energy,is directed into the cleaning liquid. A suitable acoustic range is about3 watts/cm² or less.

Following treatment with the cleaning liquid, the cleaned wafer 12 canbe processed further in any desired way, depending upon what stage ofmanufacture the wafer is at. In some instances, the substrate may besubjected to additional manufacturing steps. In other instances, thewafer may be simply rinsed and dried. Preferred rinsing and drying isaccomplished by removing the wafer 12 from the cleaning vessel andtransferring the substrate 12 to a second treatment vessel in which anSTG™ rinse/dry treatment is carried out. The equipment for performingsuch a treatment, is described in U.S. Pat. No. 5,571,337. Additionally,preferred equipment for carrying out an STG™ rinse/dry is commerciallyavailable from Yield Up International, Inc.

As an overview, the STG™ process involves first rinsing the items to berinsed and dried with a cascading flow of filtered deionized water. Thisrinse occurs in a vessel having a cascade overflow design with a blanketof low flow, hot nitrogen. The level of deionized water is then slowlydropped. A typical drop rate is on the order of 0.5 mm/s to 15 mm/s,preferably 1 mm/s to 2 mm/s. As the liquid level is dropped, the lowflow, hot nitrogen is maintained, but a flow of nitrogen containing anultradilute concentration of a cleaning enhancement substance, such asisopropyl alcohol, is also introduced. When the water level drops belowthe items being rinsed and dried, the liquid contents of the vessel arequickly dumped, e.g., at a rate of 20 mm/s. This quick dump occurs whilethe flows of the hot nitrogen and the nitrogen/IPA mix are maintained.After the quick dump of the liquid contents, the flow of nitrogen/IPA isstopped while the flow of the hot nitrogen is increased to a higher rateto dry the items, e.g., 300 liters/min to 600 liters/min. The resulantdried items can then be removed for further use, processing, storage, orthe like. For this process, the nitrogen and nitrogen/IPA are preferablyfiltered. The hot nitrogen is heated to about to 130° C. but is believedto be at a temperature of about 50° C. in the rinse/dry vessel when itenters the rinse/dry vessel through nozzles in the lid of the rinse/dryvessel.

In order to more concretely illustrate the principles of the presentinvention, FIG. 2 is a schematic illustration of a preferred system 100suitable for cleaning one or more substrates 106 using cascading flowsof deionized water, ozonated water, and/or ultradilute, aqueous ammoniaas processing liquids. The principles of system 100 may be carried outin actual practice using a Model 6200 or other Series 6000 apparatuscommercially available form Yield Up International, Mountain View,Calif.

System 100 generally includes cleaning vessel 102 contained insidehousing 104. Housing 104 may include one or more access panels such aslid 103 in order to allow access to the interior of housing 104 forpurposes of loading and unloading substrates, maintaining system 100,conducting repairs, and the like.

Cleaning vessel 102 is suitable for establishing a cascade flow of oneor more process liquids past one or more substrates 106 positionedinside cleaning chamber 108. Cleaning vessel 102 is formed from innertank 110 defining cleaning chamber 108 and outer tank 112. At leastinner tank 110, and preferably outer tank 112 as well, are preferablymade of a material such as quartz that absorbs as little acoustic energyas possible. Conventionally, substrates 106 are supported insidecleaning chamber 108 on a suitable carrier (not shown). In the case ofsemiconductor wafers, one representative wafer carrier is described inU.S. Pat. No. 5,571,337. Processing liquid enters cleaning chamberthrough inlet 114 positioned at the bottom of inner tank 110. The outletof cleaning chamber 108 is formed by rim 116. A cascading flow ofprocessing liquid introduced into cleaning chamber 108 through inlet 114thus flows upward and cascades over rim 116 into outer tank 112 fromwhich the processing liquid can be recycled or discarded as desiredthrough valved drain line 109.

The cascading flow of processing liquid can be established pastsubstrates 106 either before or after substrates 106 are positionedinside cleaning chamber 108. If substrates 106 are placed into cleaningchamber 108 before the cascading flow is established, then, as is shown,top surface 118 of processing liquid 120 will rise until processingliquid 120 fills inner tank 110 and then cascades over rim 116 intoouter tank 112. As inner tank 110 fills and top surface 118 ofprocessing liquid 120 rises, top surface 118 traverses upward and acrosssurface 107. This kind of traversal is particularly desirable whenprocessing liquid 120 is ultradilute aqueous ammonia in that cleaningperformance is much better when this traversal occurs. As noted above,the reason for this is not known with certainty, but it is believed thatelectrostatic repulsion between surface 107 and particles on surface 107are strongest at the interface between surface 107 and top surface 118of processing liquid 120. Thus, as top surface 118 of processing liquid120 moves up surface 107 of substrate 106, electrostatic repulsion isbelieved to help knock particles off the surface and 120 into the bodyof processing liquid 120 where the particles are more easily carriedaway.

Processing liquid from chemical supply line 150 is introduced into innertank 110 through sparger 122 positioned at the bottom of inner tank 110.Sparger 122 is a conduit comprising a plurality of bottom orifices 124through which processing liquid 120 is discharged into inner tank 110.By discharging processing liquid 120 downward into inner tank 110,currents and eddies against substrates 106 are minimized and laminarflow through inner tank 110 is more easily achieved. Sparger 122 ispreferably made of a material such as quartz that absorbs as littleacoustic energy as possible. Cleaning vessel 102 is positioned overmegasonic energy source 126 containing megasonic device 128 and fluidcoupling 130 through which megasonic energy is applied to the contentsof inner tank 110. The commercially available megasonic energy systemlisted above and commercially available from Kaijo Corp. because such asystem allows cleaning vessel 102 to be removably coupled to system 100so that cleaning vessel 102 independently can be easily removed formaintenance, repair, or replacement.

Optionally, although not required, it may be desirable to maintain ablanket 132 of clean, inert gas over the top of cleaning vessel 102 tominimize the exposure of substrates 106 to particles and othercontaminants in the ambient. When such a blanket 132 of inert gas isdesired, inert gas such as N₂ or the like can be introduced into volume134 above cleaning vessel 102 through gas supply line 136. The flow rateof inert gas through gas supply line 136 is controlled by valve 138.Although not shown in FIG. 2, the inert gas preferably is filtered toensure that the inert gas supplied to volume 134 is clean. The inert gascan be supplied from any suitable source, but is preferably suppliedfrom reservoir 140. Reservoir 140 contains a reserve of inert gas thatcan quickly fill housing 104 when processing liquid is dumped fromcleaning vessel 102 through drain line(s) 142. The inert gas an beexhausted from housing 104 through gas exhaust line 144. Flow of inertgas through gas exhaust line 144 can be controlled by valve 146.

Valve 148 on chemical supply line 150 can be adjusted to control theflow of processing liquid introduced into inner tank 110 through sparger122. The bottom of inner tank 110 is also fitted with one or more drainvalves 143 on one or more drain lines 142 so that the contents of innertank 110 can be quickly dumped to shorten cycle time.

One or more processing liquids are supplied to chemical supply line 150from chemical supply 152. As shown, chemical supply 152 preferablyprovides the capability of delivering processing liquids comprisingdeionized water, ozonated water, and/or ultradilute aqueous ammonia tochemical supply line 150. With respect to supplying ultradilute aqueousammonia, chemical supply 152 includes aqueous ammonia supply 154 thatprovides the capability of forming aqueous ammonia on demand fromfiltered deionized water and filtered ammonia gas in mixer 156. Water issupplied to mixer 156 through water supply line 158, and ammonia gasfrom source 169 is supplied to mixer 156 through ammonia supply line160. Valve 171 controls the flow of ammonia gas from source 169. Hotwater from source 161 is provided via hot water supply line 162, and theflow of hot water may be controlled with valve 163. Cold water fromsource 165 is provided via cold water supply line 164, and the flow ofcold water may be controlled by valve 167. The cold and hot water can besupplied to mixer 156 separately. Alternatively, appropriate ratios ofthe two water flows can be combined when it is desired to supply waterto mixer 156 that has a temperature intermediate between thetemperatures of the hot and cold water streams.

The water supplied to mixer 156 is preferably cleaned using filterdevice 166. Although any filter device could be used, the Clean Pointfiltration system commercially available from Yield Up International ispresently preferred because its combination of positive and negativesurface charged filters removes particles from the water, e.g.,particles as small as 0.05 microns, with only a minimal pressure drop.The ammonia gas from source 169 is also filtered using an appropriategas filtration unit 168. Suitable gas filtration units are standard inthe industry and are commercially available, for example, from MilliporeCorp.

Mixer 156 may be any equipment known in the art that can controllablycombine a flow of at least one liquid with a flow of at least one gas.Examples of equipment that could be used to accomplish suchgas-in-liquid mixing are widely known and would include bubblers, poroussepta, cascade systems, mechanical agitators, and the like. Aparticularly preferred gas-in-liquid mixer is commercially availablefrom Legacy Systems, Inc., Richardson, Tex. Other gas-in-liquid mixersare also described in U.S. Pat. No. 5,464,480, as well as in Chapter 18of Perry and Chilton, Chemical Engineer's Handbook, Fifth Edition, 1973.

The use of aqueous ammonia supply 154 as shown offers numerousadvantages. First, the use of gaseous ammonia as a solute, instead of aconcentrated ammonia solution, allows the user to prepare ultradiluteaqueous ammonia solutions with great precision over long periods oftime. Very accurate concentrations of ammonia in water can be easilyachieved under continuous, steady state conditions merely by controllingthe relative flow rates of the water and ammonia gas. For instance,ammonia concentrations as low as 17 ppm +/−0.05% can be easilyestablished and maintained. This kind of precision cannot be practicallyachieved by an approach in which water is used to dilute small volumesof more concentrated solutions. Indeed, it is the use of gaseous ammoniaas a solute that makes formation of ultradilute solutions repeatable andpractically feasible. Second, the processing liquid can be quicklyswitched from aqueous ammonia to just deionized water simply by turningoff the flow of ammonia gas. Thus, aqueous ammonia supply 154 alsoserves as a supply of deionized water. Third, the temperature of theaqueous ammonia or water, as the case may be, is easily controlledmerely by adjusting the relative flow rates of hot and cold watersupplied to mixer 156. Fourth, the ultradilute aqueous ammonia not onlycleans substrate 106, but it also cleans the surfaces of cleaning vessel102 and corresponding supply lines and inlet mechanisms that come intocontact with the ultradilute aqueous ammonia. Thus, system 100 is alsoself-cleaning in this regard.

Ozonated water is supplied from ozonated water supply 170. The ozonatedwater is formed by passing deionized water from valved supply line 172through ozonator 174 in which ozone is dissolved in the water to formthe ozonated supply. Valve 176 controls the flow of ozonated water tocleaning vessel 102. When an ozone generator is first started, it cantake a few moments for the ozone generator to reach steady stateconditions. It is preferred, therefore, to include bypass 178 so thatthe ozonating flow can be run continuously even when there is no demandfor ozonated water in cleaning vessel 102. Flow through bypass 178 iscontrolled by valve 180.

According to a preferred mode of operation, a cascading flow ofdeionized water is established in cleaning vessel 102. The substrates106 may be positioned in cleaning vessel 102 either before or after thecascading flow is established. The water may be at any convenienttemperature ranging from 15° C. to 98° C., but most conveniently is atabout room temperature. The flow rate of rinse water can be at anysuitable rate. Generally, a flow rate of 0.1 to 100, preferably 1 to 30,more preferably about 5, gallons per minute would be suitable. Rinsingunder these conditions for 2 to 5 minutes is generally adequate.

Next, an optional cascading flow of ozonated water past the substrates106 is established. Treatment with ozonated water is particularlydesirable when the surfaces being cleaned are hydrophobic, but can alsobe beneficial even if the surfaces are hydrophilic. The treatmentoxidizes the surfaces, making them hydrophilic. It is believed thatcleaning performance is enhanced when the substrate surface ishydrophilic. The flow rate of ozonated water can be at any suitablerate. Generally, a flow rate of 0.1 to 100, preferably 1 to 30, morepreferably about 5, gallons per minute would be suitable. Treatmentunder these flow rates at room temperature for 2 to 5 minutes would begenerally adequate with respect to ozonated water containing 5 ppm to100 ppm, preferably 10 ppm to 50 ppm, most preferably about 17 ppm

After the optional ozone treatment, substrates 106 are again rinsed withDI water. In this case, however, the rinse water is preferably heated toa temperature in the range from about ambient to about 85° C. Rinsingwith hot water for 2 to 5 minutes at flow rates of 0.1 to 100,preferably 1 to 30, more preferably about 5, gallons per minute would begenerally adequate to rinse away any remaining ozone.

After the hot water rinse is complete, a suitable flow rate ofsemiconductor grade, anhydrous ammonia gas can be turned on in order todissolve ammonia in the water and form aqueous ammonia. In preferredembodiments, the resultant concentration of dissolved ammonia (which isbelieved to actually be in the form of NH₄OH in the water) isultradilute. More preferably, suitable ultradilute aqueous ammoniasolutions are prepared by combining 0.001 ml/min to 100 ml/min, morepreferably 0.05 to 10 ml/min of ammonia (20 psi and ambient temperature)with 0.1 to 100, preferably 1 to 30, more preferably about 5, gallonsper minute deionized water. Optionally, the hot rinse water may bedumped from cleaning vessel 102 before the aqueous ammonia is introducedinto cleaning vessel 102, although cleaning performance is better if thehot rinse water is dumped first. Cleaning with the aqueous ammonia atthese flow rates and ambient temperature for 2 to 5 minutes wouldgenerally be sufficient to accomplish cleaning. After this period oftime, the substrates 106 can be pulled out of the cascading flow ofaqueous ammonia for further treatment as desired, or the aqueous ammoniacan be dumped from cleaning vessel 102 before transferring thesubstrates 106 to another treatment and/or subjecting the substrates 106to further treatment in the same cleaning vessel 102.

FIG. 2 shows system 100 which includes only a single processing vesselin which the ammonia treatment, and optionally the ozone treatment, arecarried out. However, in preferred embodiments, such cleaning treatmentsare carried out in conjunction with other treatments that may occur inthe same vessel, in the same apparatus, but a different vessel, or in adifferent apparatus. For example, the Model 6200 system commerciallyavailable from Yield Up International includes both a megasonic cleaningvessel for carrying out ammonia cleaning treatments as described aboveas well as a separate processing vessel for carrying out STG™ rinsingand drying as described in U.S. Pat. No. 5,571,337. Both the ammoniacleaning vessel and the STG™ rinse/dry vessel of the Model 6200 systemare contained in the same housing to allow both the cleaning and therinsing/drying to occur in the same controlled environment, thusavoiding transport from one piece of equipment to another betweentreatments.

A schematic representation of the Model 6200 apparatus 200 is shown inFIG. 3. Cleaning vessel 202 and rinse/dry vessel 204 are containedinside housing 206. Lids 208 and 210 provide access to each vessel.Housing 206 also includes other access panels and doors (not shown) toallow componentry to be maintained, repaired, and replaced. Cleaningvessel 202 includes inner tank 212 and outer tank 214. One or moresubstrates 205 are positioned inside inner tank 212 for cleaning bycascading flow(s) of one or more processing liquids. Processing liquidsare supplied to inlet 216 from chemical supply 218. Processing liquidsare introduced into inner tank 212 through sparger 220. Processingliquid is discharged from outer tank 214 through valved drain line 222.Alternatively, the contents of inner tank 212 can be quickly dumpedthrough one or more valved drain lines 223. Cleaning vessel 202 isoperationally positioned on acoustic energy source 224. A blanket 226 ofinert gas can be established over cleaning vessel 202 by introducing theinert gas through gas inlet 228. Gas can be exhausted from housing 206through valved exhaust line 230.

Rinse/dry vessel 204 includes inner tank 250 and outer tank 252. One ormore substrates 254 are positioned inside inner tank 250. Processingliquids are introduced into inner tank 250 through inlet mechanism 256from chemical supply 219. Drain valve 232 can be opened to quickly dumpthe contents of inner tank 250 through drain line 221. At the bottom ofinner tank 250, a baffle or mesh 234 or the like is positioned todistribute incoming processing liquid and to help foster laminar flow ofprocess liquid upward through inner tank 250. At the top, rinse/dryvessel 204 is covered by lid 236 through which process gases may beintroduced into rinse/dry vessel 204. For example, as shown, system 200is set up so that nitrogen gas or nitrogen gas containing trace amountsof a drying enhancement substance such as isopropyl alcohol (IPA) can beintroduced into rinse/dry vessel 204. In this regard, nitrogen gas issupplied from nitrogen source 238. The nitrogen can be conveyed throughvalved supply line 240 to container 242 of liquid IPA. In container 242,the nitrogen is discharged into the liquid IPA, it bubbles upward, andthen is conveyed from container 242 to rinse/dry vessel 204 via valvedsupply line 244. As the nitrogen bubbles through the IPA, the gas picksup a trace amount of the IPA as a vapor constituent. Alternatively, thenitrogen can be conveyed directly to rinse/dry vessel 204 from nitrogensource 238 via valved supply line 246. Gas supply line 249 leads to gasinlet 228. Any of supply lines 240, 244, 246, and/or 249 may be heated(not shown). Also, the gas conveyed to rinse/dry vessel 204may also befiltered (not shown) if desired.

The present invention will now be further described with respect to thefollowing illustrative examples.

EXAMPLE 1

Contaminated silicon wafers bearing a layer of native oxide werecleaned. Before the cleaning procedure, the wafers showed particlescounts of greater than 30,000 with respect to particles having a sizegreater than 0.16 microns. A Yield Up International Model 6200 apparatuswas used to carry out the cleaning procedure. This apparatus includes ahousing containing a cleaning vessel on the left side, and a STG™rinse/dry vessel on the right side. The cleaning vessel is a quartzcascade/overflow tank with a megasonic transducer liquid-coupled to thebottom of the tank. The STG™ rinse/dry tank is a standard Yield Up STG™rinse/dry tank of the type incorporated into the Yield Up Series 1000,2000, and 4000 systems.

A cassette of 50 wafers was placed into the empty cleaning vessel withthe wafers being substantially vertical as shown in U.S. Pat. No.5,571,337. A cleaning liquid containing approximately 100 ppm anhydrousammonia gas dissolved in filtered deionized water was introduced intothe bottom of the cleaning vessel at a rate such that the surface of thecleaning liquid took 300 seconds to rise from the bottom of the wafersto the top of the wafers. While the liquid level rose, megasonic energywas directed into the cleaning vessel using the system available fromKaijo; Corp. The megasonic energy level was set at “250” on the machinesdigital display. The water was filtered using the Yield Up InternationalClean Point® filtration system. The ammonia gas was mixed into the waterusing the apparatus available from Legacy Systems, Inc. The cleaningliquid temperature was maintained at 60° C.

After the 300 seconds and without dumping the aqueous ammonia, thewafers were then rinsed in the same vessel with filtered deionized waterat 60° C. for 200 seconds at a flow rate of about 5 gallons per minute.The wafer cassette was then lifted out of the rinsing water in thecleaning vessel and transferred to the STG™ rinse/dry vessel. The YieldUp International STG™ rinse/dry procedure was then carried out using thestandard STG™ process parameters provided by the manufacturer for use inits Model 1000, 2000, 4000, and 6000 Series systems.

The resultant dried wafers were than analyzed for post-clean particlecounts. The wafers showed particle counts on the order of only 32particles having a size greater than 0.16 microns, demonstrating acleaning efficiency of better than 99.9%.

EXAMPLE 2

The method of Example 1 was used to treat pristine clean prime wafers inorder to assess the effects of the cleaning procedure upon the surfaceroughness and metal contamination of the treated wafers. Surfacemicroroughness was evaluated in an area 2 micron×2 microns at the wafercenters. The results were expressed in root-mean-square (RMS), meanroughness (Ra), and peak-to-valley distance (Rmax) and are listed in thefollowing table:

TABLE 1 Surface Microroughness (angstroms) before and after cleaning.RMS Ra Rmax Pre-clean 0.8 0.6 15.8 Post-clean 0.6 0.5 7.3

This data shows that the cleaning operation had no adverse effect uponthe surface roughness of the wafers. In fact, the wafers surprisinglywere smoother after cleaning.

With regard to metal contamination, TXRF results are summarized in thefollowing table:

TABLE 2 TXRF results (10¹⁰ atoms/cm²). Location on wafer K Ca Ti Cr MnFe Ni Cu Zn Pre-clean Center <9 <6 <3 <1.5 <1.3 <1.1 <0.9 <0.7 <1.4Middle <7 <4 <5 <1.1 <1.0 <0.8 <0.7 <0.6 <0.8 Bottom <7 <6 <4 <1.5 <1.3<1.1 <0.9 <0.8 <1.0 Post-clean Center <9 <6 <4 <1.5 <1.3 <1.1 <0.9 <0.8<1.3 Middle <7 <5 <2 <1.2 <1.0 <0.9 <0.7 <0.6 <0.8 Bottom <7 <5 <5 <1.5<1.3 <1.1 <0.9 <0.8 <1.0

These results show that no added metal contamination occurred during thecleaning process as a practical matter.

EXAMPLE 3

For comparison purposes, the procedure of Example 1 was repeated, exceptno anhydrous ammonia gas was dissolved in the filtered deionized water.Thus, the wafers were cleaned only with deionized water. After cleaning,the wafers still showed a particle count of greater than 30,000 withrespect to particles greater in size than 0.16 microns.

EXAMPLE 4

A design experiment was performed in order to assess the impact thatacoustic energy, the cleaning enhancement substance, and temperaturehave upon cleaning performance. A Yield Up International Model 6200apparatus as described in Example 1 was used to carry out theexperiment.

The experiment was conducted on respective groups of 200 mm wafers inwhich three wafers of each group were intentionally contaminated withSiO₂, Si₃N₄ or W particles, respectively, using an MSP-2300 particledeposition system commercially available from MSP Corporation. Suchwafer preparation has been described by Liu, et al., Institute ofEnvironmental Sciences Proceedings, 8-16 (1999). Approximately 20,000 to25,000 particles in the diameter size range of 0.06 micrometers to 0.3micrometers were deposited over the entire surface of each wafer.Particle counts before deposition (N_(i)), after deposition (N_(d)) andafter cleaning (N_(c)) where measured utilizing a KLA-Tencor SurfscanSP1^(TBI) instrument. Particle removal efficiencies were calculated aspercent removal using the following formula:

% Removal=((Nd−Nc)/(Nd−N _(i))×100%

Four experimental runs were carried out. Run No. 1 involved positioninga cassette containing three contaminated wafers and one control waferinto the cleaning vessel of the Yield Up International Model 6200apparatus. A cascading flow of a cleaning liquid at 60° C. andcontaining about 50 ppm anhydrous ammonia gas dissolved in filtereddeionized water was established. The deionized water was filtered beforeaddition of ammonia using a Yield up International Clean Point®filtration system. A heater included with the Model 6200 apparatus wasused to heat the deionized water feed to 60° C. before the ammonia gaswas added as well.

The cleaning liquid was introduced into the bottom of the cleaningvessel at a flow rate such that the surface of the cleaning liquid tookabout 300 seconds to rise from the bottom of the wafers to the top ofthe wafers. As the liquid level rose, megasonic energy was directed intothe cleaning vessel. The megasonic energy level was set at “450” on themachine digital display. After the 300 seconds and without dumping theaqueous ammonia, the of ammonia gas flow was stopped as the wafers werethen rinsed in the same vessel with filtered deionized water at 60° C.for an additional 200 seconds at a flow rate of about five gallons perminute. The wafer cassette was then lifted out of the rinsing water inthe cleaning vessel and transferred to the STG™ rinse/dry vessel. TheYield up International STG™ rinse/dry procedure was then carried outusing the standard process parameters provided by the manufacturer foruse in its model 1000, 2000, 4000, and 6000 series systems.

Run No. 2 was identical to run No. 1 except that run No. 2 was carriedout under ambient conditions without heating the deionized water, whichwas supplied at about 20° C. Run No. 3 was identical to run No. 1 exceptthat run No. 3 was carried out without using any ammonia in the cleaningliquid. Run No. 4 was identical to run No. 1 except that run No. 4 wascarried out without using any megasonic energy.

The results of this design experiment are shown in the following tables:

TABLE 1 Particle Removal Efficiencies SiNO₂ Particles Particle Size 60°C. 20° C. No NH₃ No Meg >0.06 μm 82% 86% 76% 16% >0.12 μm 73% 81% 51%15%

TABLE 2 Particle Removal Efficiencies Si₃N₄ Particles Particle Size 60°C. 20° C. No NH₃ No Meg >0.06 μm 81% 84% 61% 32% >0.12 μm 65% 67% 25%13%

TABLE 3 Particle Removal Efficiencies W Particles Particle Size 60° C.20° C. No NH₃ No Meg >0.06 μm 91% 96% 68% 84% >0.12 μm 79% 94% 50% 82%

Surprisingly, run No. 2, which was carried out at 20° C., yielded higherparticle removal efficiencies that run No. 1, which was carried out at60° C. This result was unexpected, because most researchers havereported that particle removal efficiencies increased with an increasein process temperature when utilizing SC-1 cleaning chemistry. Thepositive relationship between expected removal efficiency andtemperature is based upon the conventional belief that particle removalis accomplished via an etching mechanism when SC-1 chemistry is used forcleaning. According to conventional wisdom, therefore, reduced particleremoval efficiencies would have been expected at 20° C. as compared to60° C. because etching rate decreases with decreasing temperature. Thefact that the ultradilute ammonia/megasonic process of the presentinvention was better when carried out at 20° C. indicates that thecleaning mechanism of the present convention is not an etching mechanismsuch as is associated with SC-1 chemistry. While note wishing to bebound by theory, it is believed that electrostatic repulsion forces, notetching, plays a significant role in the ultradilute chemistry of thepresent invention.

Without ammonia, run No. 3 demonstrates a significant decrease inparticle removal efficiencies. The results suggest that the megasonicenergy, in the absence of ammonia, might help to detach a large numberof surface-bound particles from a wafer. But then a significant numberof the dislodge particles are able to be redeposited back onto the wafersurface. While not wishing to be gone by theory, it is believed that theparticles and the wafer surfaces tend to be oppositely charged. As aconsequence, electrostatic attraction, at least in part, is causing theparticles to settle back onto the wafer surfaces.

Without acoustic energy such as megasonic energy, run No. 4 alsodemonstrates a significant decrease in particle removal efficienciesThese results suggest that the megasonic energy help to break thesurface adhesion forces between the particles and the wafer surfaces.The results also suggest that such forces are greater for SiO₂ and Si₃N₄than for W.

In summary, the results from this experiment indicate that theultra-dilute ammonia/megasonic process for the present convention reliesupon the megasonic energy to help detach surface bound particles whilethe high pH of the ultra-dilute, aqueous ammonium hydroxide cleaningliquid helps to prevent reattachment of the particles to the wafersurface. Further, the process appears to be more effective at lowertemperature, e.g., 20° C. as compared to higher temperature, e.g., 60°C.

What is claimed is:
 1. A method of cleaning a surface of amicroelectronic device at a stage of manufacture, comprising the stepsof: providing a cleaning liquid comprising solvent and ammonia, whereinthe solvent and ammonia comprise a volume ratio of solvent to ammonia inthe range from about 500:1 to about 500,000:1, and wherein the cleaningliquid does not include hydrogen peroxide; positioning the device in avessel; introducing the cleaning liquid into the vessel under conditionseffective to help clean the surface of the device, with the provisothat, if the surface of the device is subjected to an oxidizingtreatment to facilitate said cleaning of the surface, the introductionof an oxidizing agent to carry out such oxidizing treatment occurs as apre-treatment prior to the introduction of the cleaning liquid, whereinthe cleaning liquid etches 10 angstroms or less of the surface of thedevice, and wherein the cleaning liquid is at a temperature in the rangefrom about 0° C. to about 25° C. during at least a portion of the timethat the cleaning liquid contacts the surface of the device; andapplying acoustic energy to the cleaning liquid during at least aportion of the time that the cleaning liquid contacts the surface of thedevice.
 2. The method of claim 1, wherein the solvent comprises aqueousliquid.
 3. The method of claim 2, wherein the step of introducing thecleaning liquid to the vessel comprises filling the vessel with thecleaning liquid while the device is positioned in the vessel and whereinthe acoustic energy is applied to the cleaning liquid as the vessel isfilled.
 4. The method of claim 2, wherein the providing step comprisesdissolving gaseous ammonia in the aqueous liquid.
 5. The method of claim2, wherein the aqueous liquid and ammonia comprise a volume ratio ofaqueous liquid to ammonia in the range from about 1,000:1 to about300,000:1.
 6. The method of claim 2, further comprising the step of,prior to the step of introducing the cleaning liquid to the vessel,contacting the surface of the device with a processing fluid comprisingthe oxidizing agent.
 7. The method of claim 2, wherein the step ofintroducing the cleaning liquid comprises progressively immersing thedevice in the cleaning liquid, and wherein the acoustic energy isapplied to the cleaning liquid while the device is being progressivelyimmersed.
 8. The method of claim 2, wherein the aqueous liquid andammonia comprise a volume ratio of aqueous liquid to ammonia in therange from about 100,000:1 to about 200,000:1.
 9. The method of claim 2,wherein the cleaning liquid etches 5 angstroms or less of the surface ofthe device.
 10. The method of claim 4, wherein the aqueous liquidcomprises water and wherein the step of providing the cleaning liquidcomprises filtering the water with a filter system comprising positivelyand negatively charged filters.
 11. The method of claim 6, wherein theoxidizing agent comprises aqueous ozone.
 12. The method of claim 6,wherein the oxidizing agent comprises hydrogen peroxide.